In a myriad of industrial applications and processes, conventional multi-stage steam turbines may often be utilized to maintain a working fluid (i.e., steam) at varying predetermined pressures over a broad range. Energy of the pressurized steam may be extracted from the multi-stage steam turbines and converted to work to operate one or more downstream processes. A conventional multi-stage steam turbine 100 is illustrated in FIG. 1 and may include a casing 102 having a rotary shaft 104 supported therein by one or more suitable bearings (one is shown 106). In operation, the steam may be introduced into the casing 102 via an inlet control valve 108 and subsequently directed to successive zones or stages (two are shown 110, 112) of the multi-stage steam turbine 100. A first or upstream stage 110 may be configured to contain the steam at a relatively higher pressure than a second or downstream stage 112, which may be configured to contain the steam at a relatively lower pressure. The multi-stage steam turbine 100 may include a grid valve assembly 114 disposed about the rotary shaft 104 between the upstream stage 110 and the downstream stage 112. The grid valve assembly 114 may be configured to control a flow of the steam from the upstream stage 110 to the downstream stage 112. Accordingly, the grid valve assembly 114 may also be configured to control and/or maintain a pressure differential of the steam between the upstream and downstream stages 110, 112.
As illustrated in FIG. 1, the conventional grid valve assembly 114 may include a stationary plate 116 and a rotatable plate 118 disposed on or adjacent the stationary plate 116 and configured to rotate relative to the stationary plate 116. The stationary plate 116 and the rotatable plate 118 of the grid valve assembly 114 may each define a plurality of openings 120, 122 extending axially therethrough. In operation, the grid valve assembly 114 may be actuated to an “opened” position to thereby provide fluid communication between the upstream and downstream stages 110, 112. The grid valve assembly 114 may be actuated to the “opened” position by actuating or rotating the rotatable plate 118 such that the plurality of openings 122 thereof align or overlap with the plurality of openings 120 of the stationary plate 116. The grid valve assembly 114 may also be actuated to a “closed” position to thereby prevent fluid communication between the upstream and downstream stages 110, 112. The grid valve assembly 114 may be actuated to the “closed” position by rotating the rotatable plate 118 such that the plurality of openings 122 thereof do not overlap with the plurality of openings 120 of the stationary plate 116. In the “closed” position, at least a portion of the steam may be extracted from the upstream stage 110 to a downstream process via an extraction conduit 124.
As previously discussed, the grid valve assembly 114 may be configured to control and/or maintain the pressure differential of the steam between the upstream and downstream stages 110, 112 of the multi-stage steam turbine 100. The pressure differential of the steam, however, may increase resistance to or prevent the actuation of the grid valve assembly 114 between the “opened” and “closed” positions. For example, the pressure differential between the upstream and downstream stages 110, 112 may result in a net biasing force being applied axially to the rotatable plate 118, which may urge the rotatable plate 118 toward the stationary plate 116 and cause the respective annular surfaces 126, 128 thereof to engage one another. The engagement of the respective annular surfaces 126, 128 may result in the formation and/or increase of frictional forces between the rotatable plate 118 and the stationary plate 116, which may resist or prevent the rotation of the rotatable plate 118.
In multi-stage steam turbines 100 having a relatively low pressure differential (e.g., about 690 kPa or less), conventional actuators may be utilized to provide an actuating force capable of rotating the rotatable plate 118 despite the frictional forces. In multi-stage steam turbines 100 having a relatively high pressure differential (e.g., about 690 kPa or greater), however, the conventional actuators may not be capable of overcoming the frictional forces. For example, as the pressure differential between the upstream and downstream stages 110, 112 increases, the net biasing force and the resulting frictional forces may correspondingly increase, and the conventional actuators may not be capable of overcoming the increased frictional forces. Accordingly, the high pressure differential of the multi-stage steam turbine 100 may impose a limitation on the utility of the conventional grid valve assembly 114. While an increased actuating force may be employed to overcome the limitations of the grid valve assembly 114, utilizing actuators capable of providing the increased actuating force may not be a commercially and/or economically viable option.
What is needed, then, is a grid valve assembly that may be actuated in multi-stage steam turbines having a relatively high pressure differential between successive stages thereof.